Device for reduction of cross talk between different lines at switches for distribution of programs



April 19, 1949. N. v. PETERSON DEVICE FOR REDUCTION OF CROSS DIFFE 2,467,494 TALK WEEN HES F RENT LINES AT SWITC DISTRIBUTION OF PROGRAMS Filed March 27, 1947 6 @N Mm i am W m mm 5 m m E m w m S u N Patented Apr. 19, 1949 DEVICE FOR REDUCTION OF CROSS TALK BETWEEN DIFFERENT LINES AT ..SWITCHES FOR DISTRIBUTION PROGRAMS Nils Vilhelm Peterson, Stockholm, Sweden, as-

signor to Telefonaktiebolaget L M Ericsson, Stockholm, Sweden, a company of Sweden Application March 27, 1947, Serial No. 737,488 In Sweden April 1, 1946 The present invention relates to a switching device for distribution of audio frequency programs, at which electric signals are distributed from a plurality of incoming lines to at least the same number selected from a plurality of outgoing lines. The device is of that kind which consists of a contact field containing parallel conductors for one kind of lines and parallel conductors, crossing these conductors, for the other kind of lines. Communication between a line of one kind and a line of the other kind is produced by means of a Short-circuiting plug.

In spite of all precautionary measures as to choice of impedances and the like in the connected lines, impermissible cross talk arises between programs distributed by the switch, especially if only one conductor of each line appears in the contact field, the second conductor of each line being connected to ground or to a zero-point common tothe lines.

In a device of this kind the cross talk attenuation proved to be '7 nepers, which, however, is not at all sufiicient.

In order to reduce considerably this reciprocal influence between the lines there is according to this invention, an electrically reflecting plate or grid arranged in a plane parallel to the conductors of the contact field and at a distance from these which is small in relation to the length of the conductors of the contact field, said plate suitably consisting of a metal sheet or metal grid connected to an electric zero-point. The plate mainly covers the same area as the contact field. In such a device an electrical image of currentcarrying conductors appears in the plate. This image reduce the influence of one circuit on another in the system, provided the plate be relatively near to the conductors. With such a plate the cross talk attenuation was increased to between 13 and 14 nepers, which lvalue indicates that a satisfactory protection for cross talk is obtained.

An embodiment of the invention is shown schematically on the annexed drawing. A number of line pairs l-5 has one conductor of each pair connected to a common conductor 2|. The second conductor of each pair is connected to a contact strip or trolley wire, B-IO respectively, constituting the above mentioned parallel conductors for one kind of lines. In efiect these conductors are in layers. These are crossed by a second layer of parallel conductors for the second kind of lines consisting of the contact strips or trolley wires [6-20 connected to one conductor of the pairs ll-l 5, respectively, the second conductor of 3 Claims. (01. 179-78 v 2 which is also connected to the common conductor 2|.

In the figure the ungrcunded conductor of line 3 is connected to the ungrounded conductor of line 13 and the ungrounded conductor of line 5 is connected to the ungrounded conductor of line 44 by means of short-circulting or connecting links $2 or suitable plugs connecting conductor 3 to conductor. l8 and conductor it to conductor I9.

Close to the contact field consisting of the two kinds of parallel conductors a reflecting screen 23 is arranged, which suitably, by means of the conductcr 2d, is connected to the conductor 2 l, common to all the lines. The expression screen is used to include a solid or perforated plate or grid, etc.

As appears from the figure the conductors and contact strip of the lines 5-14 constitute a substantially closed loop enclosing a similar loop formed by the conductors and contact strips of the lines 3-I3. These loops produce fields, partly common to each other, induced by the line currents, which produce considerable cross talk between the lines, in the absence of the reflecting plate. The reflecting plate causes an essential reduction of the field due to the neutralizing efiect of the images.

In further explanation of the effect of the reflecting plate it may be pointed out that such a conducting surface presented normal to the axes of the loops as above described, has induced therein eddy currents each of which produces a field which opposes the field of the loop producing it.

' Thus the inductance of each loop is reduced and hence the mutual inductance which latter accounts for most of the cross-talk.

The invention is naturally not limited to the construction shown in the figure, where the different lines have one conductor connected to a common conductor. It is also applicable if both conductors of the lines terminates in a contact field,

I claim:

1. A switching device forthe transfer of audiofrequency signals from two or more of a plurality of incoming signal lines to two or more of a plurality of outgoing line comprising in combination; a conductor field composed of a layer of parallel conductors at least one of which is connected to each incoming signal line, a second and closely positioned parallel layer of parallel conductors crossing those of the first layer and having at least one connected to each outgoing signal line: connecting links joining selected ones of the first layer of conductors to selected ones of the second layer, and an electrically reflecting, grounded metal screen arranged parallel to said layers, at a distance from one of them which is small compared to the length of the conductors in said layers and of at least the same area as group of parallel conductors crossing those of the 15 first group and having each connected to one wire of each outgoing pair; a common conductor connecting all of the remaining wires of the incoming and outgoing pairs; connecting links selectively joining desired conductors of the first group to desired conductors of the second group; and an electrically reflecting metal screen connected to said common conductor and arranged in proximity to both groups throughout their full areas.

3. The device as claimed in claim 2 in which the conductors in each group are arranged in closely positioned parallel planes parallel to that of the screen.

NILS VILHELM PETERSON.

REFERENCES CITED UNITED STATES PATENTS Name Date Boemer Oct. 10, 1946 Number 

